As the Obama Administration basked in the afterglow Sunday of a historic agreement to delay the Iranian nuclear program, its potential unraveling was quickly taking shape on Capitol Hill.

Hawkish Democrats and Republicans denounced or expressed skepticism about the deal reached early Sunday morning in Geneva. The deal commits the U.S. and its allies to easing sanctions on the Iranian economy in exchange for Iran’s promise to pause its nuclear program and give international inspectors greater access to it. Both sides agreed to abide by those terms for six months.

Lawmakers in Washington, however, appear poised to break that deal by pushing for additional sanctions on the Iranian regime. The White House is warning that new congressional sanctions could jeopardize a fragile agreement that it says provides the best shot at peacefully keeping Iran from developing nuclear weapons.

Even as the Iran negotiations closed in on the temporary agreement last week, Obama was already fending off a bipartisan sanctions push on the Hill. On Tuesday the President hosted a bipartisan group of Senators to lobby them against a new round of economic penalties. With an agreement in place but talks on a long-term solution continuing, the White House faces a growing coalition pushing for tougher sanctions now.

In separate statements, top Democratic Senators Charles Schumer and Robert Menendez expressed concern about the agreement inked by Secretary of State John Kerry in Geneva. Speaking late Saturday evening in Washington, Obama issued a terse warning to the legislative branch that new sanctions would undermine the American position. “Now is not the time to move forward on new sanctions,” the President said, “because doing so would derail this promising first step, alienate us from our allies and risk unraveling the coalition that enabled our sanctions to be enforced in the first place.”

But lawmakers and congressional aides in both parties are setting the stage for a new round of sanctions that would be delayed in their effect by six months. “I expect that the forthcoming sanctions legislation to be considered by the Senate will provide for a six-month window to reach a final agreement before imposing new sanctions on Iran, but will at the same time be immediately available should the talks falter or Iran fail to implement or breach the interim agreement,” Menendez said.

Across the Capitol, House Democratic whip Steny Hoyer said Sunday on CBS’s Face the Nation that passing new sanctions with a six-month delay would be the right thing to do. And Republican Senator Mark Kirk said Sunday, “I will continue working with my colleagues to craft bipartisan legislation that will impose tough new economic sanctions if Iran undermines this interim accord or if the dismantlement of Iran’s nuclear infrastructure is not under way by the end of this six-month period.”

This approach may be too clever by half, and the White House worries that congressional distrust of Iran would become a self-fulfilling prophecy. The “sword of Damocles” approach would undoubtedly anger Iran, and would potentially nullify the painstaking agreement reached this weekend. “If there are new sanctions, then there is no deal,” Iranian Foreign Minister Mohammad Javad Zarif told NBC News on Sunday. “It’s very clear. End of the deal. Because of the inability of one party to maintain their side of the bargain.”

Congress doesn’t see it that way. “With this agreement, the role of Congress will shift from bulldog to watchdog,” said a senior Senate aide involved in Iran-sanctions negotiations. “What was to be a new sanctions bill to win Iranian concessions now becomes a Damocles to enforce this agreement and ensure it doesn’t become a never-ending first step. The President should expect bipartisan enforcement legislation on his desk before Christmas.”

The White House worked frantically Sunday to defend the agreement in calls to lawmakers on Capitol Hill. White House principal deputy press secretary Josh Earnest stopped short of issuing a veto threat on new sanctions, but said the long-standing Administration position has been to oppose new sanctions to allow space for the negotiations. “The President has been very clear that he does not believe that Congress should pass additional sanctions at this time,” he told reporters aboard Air Force One. “And that’s something that we’ve been pretty clear on for some time.”